Multi-purpose portable working machines such as chain saws, cutting tools and grass trimmers that have internal combustion engines are well known. Each of these types of machines has a working tool, such as a chain or cutting blade, which is brought to an operating rotational speed by the included engine. Since the operating tool is often close-by the operator, there is a risk of contact and an accidental injury occurring. Therefore, such machines are often equipped with a mechanical security brake for the tool, together with other security arrangements such as requiring two-hand-grip engagement by the operator in order to affect operation.
The machine is normally equipped with a centrifugal clutch that engages the tool when the engine exceeds a certain rotational speed. In normal operation, the clutch improves safety because the tool does not rotate when the engine speed is reduced below the clutch-in speed. The risk for bodily injury is therefore significantly reduced.
The machine is normally started with the throttle valve positioned in a starting position in order to ensure an efficient start-up. Because of the valve position, more air flows into the motor causing the engine rotational speed to immediately increase above the clutch-in speed of the tool when the engine catches and starts. This can present a risk because the operator will not always be holding the machine in such a way that the security arrangements provide the intended protection. Still further, as the engine speed quickly rises upon starting, the clutch-in speed will be achieved before the operator is ready for the working portion (for example, a chain blade) to begin operation.
In U.S. Pat. No. 4,553,517, an arrangement is described that is intended to work in combination with the centrifugal clutch. The arrangement works in such a way that, simultaneously with the locking of the throttle valve in starting position, a circuit, as part of the ignition system, is activated. The circuit restricts the engine speed to a level below the clutch-in speed of the centrifugal clutch. A switch is deactivated when the throttle valve is no longer in the start position and thereby allows the engine to operate normally.
One problem with this solution is that it operates using a mechanical switch. This means that in case of switch failure, the arrangement will either continuously be in a speed limiting stage or never activate the speed limitation during start-up. Another problem is that the switch, to prevent failure, has to be very reliable and therefore is expensive. A further problem is that the switch cooperates mechanically with the start position knob on the machine and consequently relies on operator manipulation in order to be active during start-up. The switch is activated when activating the start position knob, and if the machine is started with half or wide open throttle valve without activating the knob, the start security system will fail to perform as intended. Still another problem is that the design of each mechanical switch is highly dependent upon the product into which it is being incorporated since the switch must cooperate and coexist with other physical components of the including machine. This means that a special technical design must be used for each product category such as power cutters, chain saws or grass cutters. In view of these drawbacks, an object of the present invention is to solve the above-outlined problems.